jet A highly directed flow of matter, or radiation that comes from such a flow.

Julian Day (JD) Astronomers simplify their timekeeping by merely counting the days, and not months and years. Each date has a Julian Day number (JD), beginning at noon,
which is the number of elapsed days since January 1st, 4713 B.C. For instance, January 1st, 1993, was JD 2448989; January 2nd, 1993, was JD 2448990; and January 1st, 2000, was JD 2451545.
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Lagrange point One of five special points in the plane of two massive bodies orbiting one another, where a third body of negligible mass can remain in equilibrium.

Lenticular Galaxies So-called because they appear to be shaped like lenses. A lenticular galaxy has a spherically
shaped component like an elliptical galaxy, and a disk. It is generally gas poor and contains few if any young stars.

light curve The variation in brightness of a star with time. [More Info]

lighthouse model The leading explanation for pulsars. A small region of the neutron star, near one of the magnetic poles, emits a steady stream of radiation which sweeps
past Earth each time the star rotates. Thus the period of the pulses is just the star's rotation period.

light year The distance that light, moving at a constant speed of 300,000km/s, travels in one year. One light year is about 10
trillion kilometers. [More Info: Photo Album]

Local Group The small galaxy group that includes the Milky Way Galaxy, the Andromeda Nebula, and about 20 smaller galaxies.

look-back time The time in the past at which the light we now receive from a distant object was emitted. [More Info: Photo Album]

luminosity One of the basic properties used to characterize stars, luminosity is defined as the total energy radiated by a star each second, at all wavelengths.

luminosity class A classification scheme which groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class
differentiates between supergiants, giants, main-sequence stars and subdwarfs.